Ion sensitive field effect transistor with amorphous tungsten trioxide gate for pH sensing

In this study, the pH sensitive properties of the amorphous tungsten trioxide (a-WO3) thin films by rf sputtering system from a-WO3 target have been investigated. The a-WO3 thin films with 600–4750 Å thickness were deposited on the electrolyte–insulator–semiconductor (EIS) structure maintained at room temperature and a total pressure of 30 mTorr in Ar mixed O2 gas for 0.5–2 h, and we could obtain the electrical resistivity of the a-WO3 films, was about 7.8×105–4.5×109 Ω-cm. The EIS structure with a-WO3 thin films can be used to detect the ion sensitivity and can be explained by C–V curve in the different acidic buffer solutions (pH=1–7) using the C–V measurement. In addition, the a-WO3 thin films were also deposited on the double layer structure of a-WO3/SiO2 gate ion sensitive field effect transistor (ISFET), and these devices were packaged with epoxy. Then, we can obtain the shift of the linear region threshold voltage (ΔVT) of the ISFET devices in the acidic solutions (pH=1–7). The a-WO3 materials exhibited a fairly high response, and the sensitivity was about 50 mV/pH.